In the art of data communications, bit signals are transmitted in serial fashion or in parallel fashion, along transmission lines, at very high speeds, and at relatively low voltage amplitudes, all of which characteristics give rise to the occasional losing of one or more bit signals. Noise factors also enter the picture and provide a basis for bit signal loss. In systems where the interface cards are changed by the user, while the system is operating, there often occurs the losing of bit signals because the card swap so often gives rise to signal transients. For all of the foregoing and for other reasons it has been necessary, in data communications systems, to do parity checks as information signals are transmitted from one point in the system to another point in the system.
In prior art data communication systems there is a provision by which the programmer can program the data processor, used in the data communication systems, to be alerted by a parity error signal and to use that alert signal to resend the message or to request that a message be resent. Such an arrangement is useful but it does require that memory space be allocated for the program information necessary to effect such a routine and it has required that either whole messages be resent or portions of messages between flags be resent both of which procedures are costly in time.
The present system adopts the philosophy that an irregularity, in data (or address) information transmission, is merely a temporary condition and that a retransmission of that same data (or address) information is likely to be correct. Accordingly the present system provides circuitry and hardware to automatically effect a "retry" when the transmission of data (or address) information signals has been determined to be incorrect.